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Quicklime is produced through the thermal processing of limestone in industrial kilns. During quarry operations, fine particulate quarry dust adheres to limestone lump surfaces, increasing the bulk concentration of impurities in limestone products. During thermal processing in a kiln, impurities such as Si, Mg, Al, Fe, and Mn react with Ca, reducing quicklime product quality. Which reactant phases are formed, and the extent to which these result in a reduction in quality, has not been extensively investigated. The present study investigated as-received and manually washed limestone product samples from two operational quarries using elemental compositions and a developed predictive multi-component chemical equilibrium model to obtain global phase diagrams for 1000–1500 °C, corresponding to the high-temperature zone of a lime kiln, identifying phases expected to be formed in quicklime during thermal processing. The results suggest that impurities found on the surface of the lime kiln limestone feed reduce the main quality parameter of the quicklime products, i.e., calcium oxide, CaO (s), content by 0.8–1.5 wt.% for the investigated materials. The results also show that, in addition to the effect of impurities, the quantity of CaO (s) varies greatly with temperature. More impurities result in more variation and a greater need for accurate temperature control of the kiln, where keeping the temperature below approximately 1300 °C, that of Hatrurite formation, is necessary for a product with higher CaO (s).

This paper reports on results from detailed studies on coal ash and limestone interactions during calcination. Industrially produced quicklime from a coal-fired rotary kiln was analyzed and compared with laboratory-scale studies of surface interactions between two coal ashes and limestone. Exposure tests were performed at 1,100 °C and 1,350 °C, in a high CO2 atmosphere. SEM-EDX analyses of the ash-quicklime interface were performed to detect and quantify changes in microstructure, as well as the depth of ash interaction into quicklime. Stable phases in the ash-quicklime interface were assessed by multi-component chemical equilibrium calculations based on local EDX analysis. The industrially produced quicklime showed intrusion by extraneous elements, mainly Al and Si, up to 800 µm into the quicklime, in accordance with expected ash composition, based on the ash analysis of coal fuel used. In laboratory-scale and 1,100 °C, ashes appeared solid to a large extent, and no distinctive microstructure difference of quicklime was observed underneath the ash-quicklime interface. At 1,350 °C, the ashes appeared molten to a large extent, and the quicklime microstructure was affected compared to at 1,100 °C, resulting in densification. For both temperatures and both coal ashes, the interface reactions reduced the amount of reactive CaO, thereby resulting in a decrease in product quality. The laboratory methodology was shown to be useful to increase mechanistic understanding of the ash-quicklime interactions. The method could be expanded to test other limestone qualities and fuels, e.g. renewable biofuels.

Net CO2 emissions from the production of quicklime can be reduced by introducing renewable solid fuels or sustainably produced electricity for heating of the process. This paper reports the results of a study examining the effects of new heat sources on quicklime surface reaction products and on quicklime microstructure. Limestone was heated to 1100 °C and 1350 °C in high CO2 atmosphere under three conditions: i) an ash mixture representing conventional coal and a solid biofuel (olive pomace); ii) olive pomace ash, and iii) no ash representing an electrically heated process. The ash-quicklime interfaces of the samples were analyzed for elemental composition and microstructure using SEM-EDX. Multi-component chemical equilibrium calculations were used to assess the stable chemical phases in the interface. Coal-olive pomace ash mixture resulted in coarsening of the quicklime microstructure; this effect was less severe compared to that of pure olive pomace ash. The calculations indicated that the potassium in olive pomace ash was bound to Si- and Al-rich coal ash phases. Exposure to potassium-rich olive pomace ash resulted in severe coarsening of the quicklime microstructure. The difference was most obvious at 1,350 °C, and was probably the result of intrusion of a potassium-rich salt melt. For limestone without ash, the quicklime showed enhanced sintering and reduced porosity at the higher temperature, in agreement with previous studies. Interface reactions and microstructure coarsening, here most apparent for the case with olive pomace, could be problematic in industrial quicklime production since they may contribute to decreased available CaO and reactivity.